1. Field of the Invention
The present invention relates to a digital data processing circuit.
2. Description of the Related Art
Recently, an FM (Frequency Modulation) transmitting circuit may be used to reproduce music data stored in a portable music reproducing device, etc., on a car stereo, for example (see, e.g., Japanese Patent Application Laid-Open Publication No. 2006-262521 or 2007-88657). FIG. 6 depicts a common configuration of a transmitting device 200 using an FM transmitting circuit 300 and a microcomputer 310 for transmitting an audio signal.
A frequency of a carrier in the FM transmitting circuit 300 is determined with consideration given to the frequency of an FM radio, etc., being used in its vicinity. Therefore, firstly, a user needs to set the frequency of the carrier in the FM transmitting circuit 300. Specifically, the user operates a key (not shown) of a controller 220 so that the frequency of the carrier displayed on a display screen (not shown) of the controller 220 will be a desired frequency. When the frequency of the carrier has been determined, the user then operates a key (not shown) of the controller 220 so that frequency data of the carrier will be output to the microcomputer 310. The microcomputer 310 transmits the frequency data output from the controller 220 to the FM transmitting circuit 300 as serial data SDA in synchronization with a clock signal SCL. As a result, the frequency of the carrier is set in the FM transmitting circuit 300, and the FM transmitting circuit 300 becomes capable of transmitting audio signals RIN and LIN to be input thereto to the car stereo by way of an antenna 230, for example.
When, in the above transmitting device 200, environment in the vicinity changes and a reproduced sound of the car stereo is affected by the FM radio, for example, it is necessary to change the frequency of the carrier of the FM transmitting circuit 300. In changing the frequency of the carrier, the audio signals RIN and LIN, which are outputs of an audio reproducing device 210, are temporarily stopped and the controller 220 is operated in general. For this reason, the setting of the FM transmitting circuit 300 is changed in a soundless state where no reproduced sound is output from the car stereo. Even in the case where the sound signals RIN and LIN are temporarily stopped while changing the setting, however, as a matter of fact, there is a case where an influence of a harmonic, etc., of the serial data SDA appears in an audible range so that a sound of transmitting the serial data SDA is reproduced as a noise by the car stereo. When the above noise is reproduced, the user may possibly suspect that the transmitting device 200 has a trouble, etc.